P621: SUBCLONAL EVOLUTIONARY TRAJECTORIES IN IGLV3-21R110 CHRONIC LYMPHOCYTIC LEUKEMIA
Authors: L. Paschold, D. Simnica, R. Benitez Brito, C. Schultheiss, T. Zhang, C. Dierks, M. Binder
Journal: HemaSphere (2022)

Background: There is a large body of evidence supporting the importance of antigenic interactions of the B cell receptor (BCR) with self or non-self antigens that make this receptor and its signaling machinery one of the most important drivers in many types of non-Hodgkin lymphoma. This has led to a new pathobiological understanding of these diseases as well as novel therapeutic strategies. Chronic lymphocytic leukemia (CLL) is a prototype disease in which BCR-BCR self-interactions induce autonomous signaling. While virtually all CLL BCR are prone to autonomous signaling induced by such contacts, the IGLV3-21R110 molecular interaction found in 10% of CLL patients defines a distinct clinical subset of patients that show a very aggressive clinical course despite novel treatments. In these cases, a single amino acid replacement from G to R located at the boundary of variable and constant region of the BCR is an important tumor promotor driving neoplastic B cell growth. Aims: It has been unclear how the R110 point mutation is generated. We set out to investigate the trajectory of the malignant CLL B cell clone and subclonal heterogeneity in IGLV3-21R110 cases in order to gain insight into the molecular pathogenesis of this unique amino acid exchange. Methods: We screened an initial CLL cohort of 127 cases by flow cytometry using an anti-IGLV3-21R110 antibody. Positive cases were subjected to immunosequencing of the light chain locus via amplicon based next-generation sequencing and characterized for typical chromosomal deletions and driver gene mutations. Results: We identified twelve CLL cases with IGLV3-21R110. Of these, we found a higher frequency of mutations in ATM (42%) and SF3B1 (25%) than in unselected CLL cohorts, which is in line with findings reported by others. As expected, the repertoire of healthy B cells in the blood of IGLV3-21R110 CLL patients contained IGLV3-21 light chains with the wild-type G110 residue, indicating that the R110 exchange is of acquired nature. Interestingly, in half of the IGLV3-21R110 CLL cases we found additional clones with IGLV3-21 and R110, including cases with divergent J gene cassettes originating from different pre-B cells. This indicates that the R110 exchange may occur multiple times within independent B cell clones in the same patient. Due to the unique location at the variable-constant boundary and the high correlation of the IGLV3-21R110 patient subset with mutations of genes involved in DNA repair (ATM/SF3B1), R110 is likely generated as a result of incorrect V-J recombination facilitated by the error-prone-environment. Summary/Conclusion: Our work indicates that the G110R point mutation results from incorrect V-J rearrangements in the error-prone environment that may be induced by SF3B1 and ATM mutations associated with this CLL subset. We found evidence for a branching light chain evolution that leads to a striking convergence of separate clones on the identical light chain amino acid exchange in the same patient. This strongly emphasizes the role of this mutation as a tumor promotor involved in malignant transformation of this subset of CLL.